Shape of water-air interface beneath a drop on a superhydrophobic surface revealed: Constant curvature that approaches zero

The 3D interface between a sessile water drop that is found on a superhydrophobic microtextured surface has been directly imaged using a confocal microscope with an immersion lens. The local mean curvature of the water-air interface was derived and was shown to be constant and close to zero not only for the case of pure water, but also for cases of variable drop density and surface tension. Although the mean curvature is constant and close to zero, the standard deviation on the mean curvature increases with increasing drop density and lower surface tension. The resulting 3D image of the water-air interface at the bottom of a water drop demonstrates the possibility of investigating practical interfaces of water on given textures and confirms that no matter what the superhydrophobic surface characteristics are, the interface remains with a constant curvature of close to zero.